1. Field of the Invention
The invention relates to an attachment lens detachably incorporated between a main camera lens and a camera such as a television camera equipped with a color separation optical system and/or filters and a video camera equipped with a color separation optical system and/or filters which imposes restraints on optical dimensions such as a back-focal distance for extending the focal length of the main camera lens, and, more particularly, to an oscillation isolating attachment lens detachably incorporated between a main camera lens and a camera such as a television camera and a video camera which prevents or significantly reduces tremors or oscillations of an image from occurring due to camera shakes.
2. Description of Related Art
With an increased demand of high resolution for the entire area of a charge coupled device (CCD) installed as an image forming device in cameras such as a TV camera and a video camera, the camera lens is required to have optical performance to form the whole area of image with a high and uniform resolution. Further, there have been put to practical use various types of attachment lenses which extend the focal length of a camera lens with which the attachment lens is used together without shifting the focal point of the camera lens. One of such attachment lenses such as known from, for example, Japanese Unexamined Patent Publication No. 63-201624 is equipped with a damping mechanism for preventing tremors of an image from occurring due to camera shakes. This attachment lens is of a type of extender having a magnification greater than approximately 1.3 which moves the whole lens groups or at least one of the lend groups in a direction perpendicular to an axis of the lens so as to cancel a tremor or oscillation of an image due to a camera shake. Japanese Unexamined Patent Publication No. 6-189181 teaches a concept that a damping device is incorporated in an attachment lens between a negative power front lens group and a positive power rear lens group and, however, does not in any way describe details of the damping device.
Typically, photographing is made in multifarious conditions and circumstances, For example, there are great demands for taking a photograph in subdued lighting or with a reduced depth of field with an effect of improving image quality. In such a case, it is essential to used a taking lens having a large aperture ratio. TV camera lenses and video camera lenses which are required to have a relatively high resolution power are essential to have a long back-focal distance because TV cameras and video cameras incorporate a color separation optical system and/or a filter assembly in front of an imaging device therein. Such a TV camera lens or a video camera lens is generally required to have a relatively long distance to an exit pupil. If the exit pupil is at a short distance, there occurs color shading due to a difference in incident angles of principal light rays incident upon upper and lower margins of a dichroic mirror of the color separation optical assembly, which is a matter of great concern for lenses having large aperture ratios. Therefore, an oscillation isolating attachment lens of this type detachably incorporated between a main camera lens and a TV camera or a video camera is essential to have a long distance to the exit pupil as well as a long back-focal length.
The extender lens described in Japanese Unexamined Patent Publication No. 63-201624 has a relatively large magnification, and hence the whole optical system of the extender lens unavoidably has a large F-number, i.e. a low speed. If the extender lens has a magnification between approximately 1.1 and 1.3, the optical system is difficult to have a high negative power and, in consequence, impossible to meet the requirement of having a long back-focal length and a long distance to the exit pupil.